Field effect transistors, such as a thin film transistor (TFT), are widely used as the unit electronic device of a semiconductor memory integrated circuit, a high frequency signal amplification device, a device for a liquid crystal drive, or the like, and they are electronic devices which are currently most widely put into practical use. Of these, with significant improvement in displays in recent years, in various displays such as a liquid crystal display (LCD), an electroluminescence display (EL) and a field emission display (FED), a TFT is frequently used as a switching device which drives a display by applying a driving voltage to a display device.
As a material of a semiconductor layer (channel layer) which is a main component of a field effect transistor, a silicon semiconductor compound is used most widely. Generally, a silicon single crystal is used for the high frequency amplification device and the device for integrated circuits which need high-speed operation. On the other hand, an amorphous silicon semiconductor (amorphous silicon) is used for a device for driving a liquid crystal in order to satisfy the demand for realizing a large-sized display.
A thin film of amorphous silicon can be formed at relatively low temperatures. However, the switching speed thereof is slow as compared with that of a crystalline thin film. Therefore, when it is used as a switching device which drives a display, it may be unable to follow the display of a high-speed animation. Specifically, amorphous silicon having a mobility of 0.5 to 1 cm2/Vs could be used in a liquid crystal television of which the resolution is VGA. However, if the resolution is equal to or more than SXGA, UXGA and QXGA, a mobility of 2 cm2/Vs or more is required. Moreover, if the driving frequency is increased in order to improve the image quality, a further higher mobility is required.
On the other hand, a crystalline silicon thin film had problems that, although it has a high mobility, it required great energy and a large number of steps in the production and formation of large-area display was difficult. For example, when crystallizing a silicon-based thin film, laser annealing which is conducted at a high temperature of 800° C. or higher and requires expensive equipment is required. Moreover, since the device configuration of a TFT was usually limited to the top-gate structure, reduction in cost, such as reduction of the number of masks, was difficult in the case of a crystalline silicon thin film.
Generally, fabrication of an oxide semiconductor thin film is conducted by sputtering using a target (sputtering target) which consists of an oxide sintered body. For example, an oxide semiconductor thin film obtained by doping indium oxide with aluminum has been disclosed (Patent Document 1). In the examples of Patent Document 1, an oxide semiconductor device is fabricated by using a target in which the atomic ratio of indium and aluminum Al/(Al+In) is 0.005. However, evaluation on target performance and studies on nodules which are generated at the time of sputtering were not sufficient.